Storm of Zehir isn't quite as fulfilling to play as NWN2 and MotB, but it is still a decent expansion.
One of the most significant of these new features is the overland map, which is a large component of SoZ. It is a good prototype for future installments of NWN. Although the developers did not exploit it to its fullest, the overland map has the potential to be as good as that of "Heroes of Might and Magic." SoZ's implementation of the overland map is a good start. It makes travelling from town to town a little more interactive and realistic than it was in the first two NWN2 games. You actually have to walk/run your party through the wilderness to get from place to place, but you get the opportunity to find hidden locations, fight groups of baddies (for more XP), and engage in unique encounters that are sometimes part of quests.
One gripe I did have was the way encounters occurred on the map. The way the game is programmed, it has to reload the normal interface and map data every time you hit a group to fight with. On my computer it's not such an issue, but on a laptop or slower desktop, the constant loading could become annoying.
The reason I like NWN is the long, epic storylines mixed with the broad character customization. SoZ's plot was not as appealing as those from other NWN games, but SoZ does add in new classes, and two new races for characters. You can can also have up to 3 fully customizable characters in your group that you design before the game starts. During the game you can add as many as three additional characters of varying classes to augment your group. You can customize the in-game characters with items, spells and levels just as in NWN2 original. Unfortunately, a large party becomes its own stumbling block in SoZ, because experience is distributed between characters in the same fashion as all other NWN games. So, with six characters, you're probably going to end up with six underpowered characters near the end of the game, unless you go power-levelling in the overland map.
I played through SoZ using one character to handle all the dialog, and not much else. I liked the game's dialog because it has a real affect of how your quests resolve, i.e. it really is open ended. However, I think I would miss out on a lot of what the game had to offer if I had gone with a combat oriented group. The story would have been shorter and a lot more linear if I didn't have a lot of skill points allocated to character interaction skills.
Another gripe I had about SoZ as a whole is, what I perceived to be, a lack of unique items in the game world. I think it's really disappointing when I have to reload my game three or four times to win a tough battle, then all I get out of it is a +3 full plate and a +2 Kukuri, instead of a +5 Fiery Plate Mail of Epic Doom(!) and a matching Sword of Supreme Awesomeness(!!). I noticed it happened more in SoZ than in other NWN games and it kind of took the fun out the random dungeon crawls. SoZ does have a large number of unique items, but a lot of them come from merchants (not much fun) and many of those same unique items don't really improve much on the regular +2 or +3 versions of the items. The lucrative dungeon crawling that I fell in love with in the other NWN games wasn't as good in SoZ.
But going back to some positives... the music is great in SoZ. In my opinion SoZ probably has the best musical score yet for NWN. It fits the atmosphere in the game very well. I also didn't feel compelled to turn off the music just because it was repetitive. It really is nice to listen to. On occation the music will stop between songs or between repeats, and I think that helps to keep it from seeming so repetitive while you're in a specific area.
The graphics in SoZ are stellar. About the only feature they didn't include was full HDR. The textures are excellent, the character models are excellent, character faces look very good up close. There's water refLection and refRaction, light blooms that don't look ridiculous, and very good shadows (except on the overland map - it has some issues). Basically, SoZ will put your high end video card to work if you have one (or two). Apparently, the game code is optimized for lower-end systems, according to the manual introduction. The game engine does start to show signs of age when a lot of stuff is going on. I have an Intel Quad Core Q9450, 4GB of DDR3 RAM and a ATI/AMD HD4870 and the game would still become sluggish when I think it should not have. Normally, it is still playable so I may be nit-picking on that issue.
I played most of SoZ with patch version 1.22. I think it could still do with a big patch for the primary code and for the AI algorithms. The hiccups and bugs in SoZ are very obvious, so I think they should have been addressed before now (May, 12.) None of the bugs are game killers, that I've noticed. They usually involve rediculous pathfinding decisions by your party members, or NPCs not having enough sense to open a door to attack. There's also a lot of popping around while moving, a lot like the "rubber-banding" internet lag you'd see in MMORPGs. That doesn't make any sense to me. Again, these aren't "throw up your hands and quit" bugs, they're just annoying when they occur.
I gave Neverwinter Nights: Storm of Zehir a 7.5 rating because it kept me entertained and never got boring or repetitive. I think it's a worthy expansion for NWN2, even with its problems, because the developers did add a lot of new stuff to the NWN2 game code. The problems are just growing pains for the game, and I'm confident future patches can address them if Obsidian and Atari are interested in improving their products.